1. Field of the Invention
The present invention relates to a method of projection exposure of a circuit pattern onto a photosensitive substrate and an apparatus therefor.
2. Related Background Art
A projection exposure apparatus, for example a stepper, employed in a so-called photolithographic step in the manufacturing process of semiconductor devices, projects and focuses the image of a circuit pattern formed on a reticle (or a photomask) onto a photosensitive substrate (wafer) through a projection optical system.
In the conventional projection exposure apparatus, the light amount distribution of the illuminating light is substantially uniform, on a plane corresponding to the Fourier transformation of the reticle pattern or on a plane in the vicinity thereof (namely on the pupil plane of the illuminating optical system), in a substantially circular or rectangular area containing the optical axis of the illuminating optical system. Such illumination is called ordinary illumination. For transferring a finer pattern in the conventional projection exposure apparatus utilizing such ordinary illumination, it has been necessary to select either the light of a shorter wavelength or the projection optical system of a larger numerical aperture.
Also for this purpose there has been proposed the so-called phase shift reticle which displaces the phase, by .pi., of the light transmitted by a specified portion of the translucent area of the circuit pattern on the reticle, with respect to the light transmitted by another portion, as disclosed for example in the Japanese Patent Publication No. 62-50811.
Also there has been proposed a method of limiting the light amount distribution of the illuminating light on the pupil plane of the illuminating optical system to a specified shape not containing the optical axis of said illuminating optical system. Said method is designed to increase the resolving power or the depth of focus by limiting the light amount distribution on the pupil plane of the illuminating optical system to an annular shape (hereinafter called annular illumination) or to a specified eccentric shape with respect to the optical axis (hereinafter called modified light source). The modified light source was reported in the 1991 Fall Convention of Applied Physics, and is equivalent, in principle, to an inclined illumination to the reticle pattern.
In the field of memory devices, the major trend of the circuit pattern is to utilize formation of stacked capacitors formed on the wafer surface. The capacity of such stacked capacitors increases with the thickness (or height) thereof, enabling attainment of a higher level of integration. In such configuration, however, there is inevitably developed a large step (in the order of 0.8 to 1 .mu.m) between a memory cell area in which such stacked capacitors are provided and the other peripheral circuit area. Because additional circuits have to be formed on such memory cell area and on the peripheral circuit area, there is required a very large depth of focus.
In the above-mentioned conventional technology, the reduction of the wavelength of the illuminating light is currently considered difficult, for example because of the absence of a suitable optical material usable for the transmissive optical elements. Also the increase of the numerical aperture of the projection optical system is quite difficult, and, even if it is made possible, there will result a drastic reduction of the depth of focus.
On the other hand, the phase shift reticle is still associated with various technical difficulties, such as its high cost arising out of the complex manufacturing process thereof, still unestablished methods of inspection and repairing, and limitations on the usable photoresist.
On the other hand, the modified light source does not increase the depth of focus for an isolated pattern or a periodical pattern in certain directions, thus hindering the higher level of integration in such memory devices utilizing the stacked capacitors.